In order to provide an efficient kneading operation, a known rotary kneading screw used in an extruder has a kneading construction as disclosed in U.S. Pat. No. 4,639,143 for example. This rotary screw includes a kneading section forming a maximum outside diameter of the rotary screw. The kneading section defines a plurality of kneading recesses having a constant depth and shape along a material transport direction. A material mixture is kneaded in the course of transportation under a constant pressure.
The above rotary kneading screw, however, has a problem due to the application of a constant pressure to the material mixture passing through the kneading section. That is, when mixing and kneading a plurality of resins in pellet form, for example, the materials are heated by heaters or the like into a highly viscous half-melted state, and tend to adhere to inside surfaces of the kneading recesses during the transportation.
Further, if the resin materials stagnate and remain in low pressure regions of the kneading recesses, it becomes difficult to transport and uniformly knead the materials by means of the constant pressure. This results in the problem of extrusion-molding the materials without kneading the materials uniformly.